1. Field of the Invention
Embodiments of the invention relate to a spray material for thermal coating of a substrate, in particular, for thermal coating a running surface of a cylinder of a reciprocating piston combustion engine. Embodiments of the invention also relate to a thermal spray layer, as well as to a cylinder with a thermal spray.
2. Discussion of Background Information
Coatings provided by thermal spraying have been known for a long time for a plurality of applications. Thus amongst other things, for example, surfaces of oil lubricated cylinder running surfaces of vehicle engines have been coated for some time by using plasma spraying. In particular, the layer significantly reduces the coefficient of friction between the piston rings and the cylinder wall so that the wear of the piston rings and cylinder is significantly reduced, which leads to an increased running life of the engine and an increase in the period between maintenance operations, for example, an oil change, and not least to a significant improvement of the engine performance.
This is achieved in the prior art by different measures. For example, such layers for oil lubricated combustion engines can include admixtures of solid lubricants in a basic matrix. The basic matrix can be provided with additional pores of pre-settable sizes which act as oil pockets and, together with the relatively soft admixed solid lubricants, significantly reduce the friction between the piston rings and the cylinder wall. The basic matrix itself, which among other things in particular includes the solid lubricants and the pores, is in this respect composed of a hard matrix material. The basic matrix ensures a long lifetime of the cylinder running surfaces and the piston rings. Such a modern high performance cylinder running surface is described in detail, for example, in EP 1 340 834, the disclosure of which is expressly incorporated by reference herein in its entirety.
Further typical applications for surfaces provided by thermal spraying are the coating of turbine parts with wear protection layers and thermal insulation layers of components of oil lubricated bearings, such as e.g. the coating of crankshafts or other work pieces which are subjected to particular physical, chemical or thermal loads. Depending on the function the layer has to perform, certain types of materials are used, which are generally in the form of spray powders or spray wires, which possess the required specific properties and composition, to generate the required properties of the surface layer to be sprayed.
For larger production volumes, the price of the powder material plays an important role with regard to the economic efficiency of the coating, particularly, the coating of cylinder running surfaces by the plasma spray method APS, such as in the case of coating larger engines (e.g., a diesel truck).
The production costs of the powder are dependent on the price of raw material and on the processing requirements required to work the raw materials into a viable material that is suitable for carrying out the selected method.
Utilizing the known method of gas atomization of metallic materials (by gas or water), a reduction of the energy costs can practically only be influenced by an improved powder yield. In this respect, the specification of the distribution of the particle size plays a pivotal role. Using the best conditions, the production costs of metallic powders in a quality, such as is required, e.g., for internal coating of cylinders for combustion engines, can nowadays hardly be reduced below US $ 10 per kg. For this reason, it is to be expected that a further cost reduction is subject to certain boundaries.
However, the performance requirements of the spray materials will increase with time. In particular, the tribological properties of the coating will become even more important with increased temperatures, since the effect of the lubricant significantly reduces with the increase in the wall temperature. In principle, tribological solutions that are applicable at wall temperatures of up to 350° C. are possible. In this respect, the anti-scuffing properties of the layer materials play a pivotal role.
As a cost-effective production method of, in particular, ceramic powders and/or of non-metallic powders for thermal spraying, generally grading and filtering can be used, even in the case of larger amounts of ceramic spray from metallic oxides. In the case of certain materials, minerals can be used in the powder without additional smelts taking place.
As a potential material for cylinder running surfaces, it was previously known to use iron titanate FeTiO3 which is also known as ilmenite. Ilmenite is formed of approximately 53% TiO2 and 47% FeO and crystallizes in a hexagonal crystal system. The hardness of ilmenite crystals is approximately 650 HV, this means that values of 400 to 500 HV are possible in the layers for optimized parameters.
For this reason an ilmenite spray material for the formation of a corrosion-resistant coating by thermal spray process was already suggested in UA 74 987, the disclosure of which is expressly incorporated by reference herein in its entirety. In WO 2004/106711, the disclosure of which is expressly incorporated by reference herein in its entirety, the applicants suggest ilmenite in part in combination with other metal ceramic materials and/or oxides as a spray material for the coating of cylinder running surfaces of supercharged engines. However, these coatings are not designed for the increased tribological requirements of highly and/or strongly fluctuating temperature loads, but primarily to improve the hardness and/or corrosion resistance of the coated surfaces.
Starting from this prior art the applicant already suggested a significantly improved spray material on an iron basis for the thermal coating of running surfaces of cylinders of reciprocating piston combustion engines with FeTiO3 as a base material in PCT/EP2009/058565, the disclosure of which is expressly incorporated by reference herein in its entirety. In this respect the improved spray material in accordance with PCT/EP2009/058565 includes at least one first solid lubricant comprising sulfide and a second solid lubricant comprising fluoride.
It could be demonstrated for the first time in the above-noted application that spray materials on an iron titanate basis, i.e., on the basis of the so-called ilmenite with the chemical formula FeTiO3 are particularly well suited, in particular for the thermal coating of combustion engine components, when the ilmenite is admixed with at least a sulfide and a fluoride as a solid lubricant. In this respect, the layers produced thereby are characterized, in particular, as having an excellent consistency with regards to the adhesion wear. Beside the addition of sulfide and fluoride to the solid lubricants, in particular, e.g., also additionally a nitride can be added, which among other things allows a significant increase in the wall temperature of the cylinder running surface in the operational state so that these coatings are also particularly well suited for use in adiabatic engines.
Through the simultaneous use of at least one sulfide and a fluoride in the spray material of PCT/EP2009/058565 it could be ensured that the thermally sprayed layers respectively have comparably good tribological properties for different temperature regions.
The tribological performance requirements of the iron titanate FeTiO3 layers (ilmenite) can be significantly improved through the targeted addition of solid lubricants. The properties of these lubricants rely among other things on the special crystal structure and the low tendency to chemically bond and/or react with metallic and ceramic materials. The precise class of solid lubricants is selected in accordance with the invention in dependence on the expected temperature loads. In the case of cylinder inner surfaces in combustion engines, advantageously the highest wall temperature, e.g., in the contact zone between the cylinder running surface and the piston rings is considered.
The solid lubricant on a sulfide basis, for example MoS2 and/or WS2 can be used in an oxidized atmosphere without problems up to a temperature of 350° C. In the case of impact loads in combustion engines, the hot contact points, however, can be formed, e.g., between the cylinder running surfaces and piston rings, such that the local temperature can be significantly higher than 350° C. For this reason, at least one further type of solid lubricant is used in accordance with PCT/EP2009/058565 which has an increased temperature durability and simultaneously is also durable in the aggressive chemical conditions in the combustion space and additionally positively influences the adhesion requirements and the hardness of the coating.
In this respect, PCT/EP2009/058565 also teaches that, beside fluorides, also solid lubricants on a nitride basis, e.g., hexagonal BN or CrN, can be used particularly advantageously, as these also achieve the function of the solid lubricants up to the highest temperatures of 950° C. also under oxide conditions. Such high temperatures frequently only appear locally, for example, in cylinders of combustion engines.
In EP 1 790 752 A1, the disclosure of which is expressly incorporated by reference herein in its entirety, a thermal spray material with a very high zinc content of at least 70% zinc is suggested that can only be sprayed onto the substrate in certain low pressure conditions of less than 100 mbar, preferably also only between 1 mbar and 10 mbar gas pressure in a process chamber and maintaining very large spray distances of at least 400 mm to the substrate. In this respect, the spray material of EP 1 790 752 A1 and the therein suggested spray process serves to replace the galvanic zinc process, which is regarded as harmful to the environment, in the area of corrosion protection. For this reason, the zinc content must be at least 70% so that a sufficient effect of the zinc coating against corrosion is achieved. Due to the high vapor pressure of zinc, the spray material of EP 1 790 752 A1 can only be successfully used in combination with the low pressure method also suggested in this document, which naturally requires the use of a closed process chamber in which the required low pressure conditions are settable. For this reason the process chamber must have an adequate size so that a minimum spray distance to the substrate to be coated of at least 400 mm is settable. Furthermore, not only the pressure plays an important role in the process chamber, but a pressure ratio of approximately 1 to 40 between the pressure in the interior of the coating jet and the actual gas pressure of the gas atmosphere has to be set in the pressure chamber, i.e., the pressure within the coating jet must be larger than the pressure of the gas atmosphere in the process chamber. This selection of the pressure parameters is also referred to in the prior art as “under expanded condition”. It is an essential recognition of EP 1 790 752 A1 that spray materials which include a material with a comparably high gas pressure such as, for example zinc, have to be sprayed with the method described in EP 1 790 752 A1, if it should be prevented that the material vaporizes to a high degree with the high vapor pressure on thermal spraying and therefore is no longer contained or is no longer sufficiently contained in the sprayed layer.
For this reason alone, pure zinc as a spray material additive will not be chosen by a person of ordinary skill in the art for thermal spray processes that are not carried out in a process chamber in a low pressure atmosphere, e.g., for inner coatings of cylinders with rotating spray gun. Additionally, the coatings of pure zinc do not have the required mechanical hardness and/or temperature durability for the application as cylinder running surfaces.